Has the hospital done any EEG testing? They can usually get a good idea of brain condition with an EEG.

If his heart was stopped for too long, the brain damage could be severe and irreversible. Only the doctors can answer this question for you.

The answers could be

He has very good EEG tracings and a good prognosis.
He has weak EEG tracings and if and when he regains consciousness, he will have disabilities due to brain damage.
Or, His EEG tracings show that his brain is damaged beyond any ability to have a conscious understanding of his life.

My heart goes out to you as you search for answers.